In recent years, with the wide spread use of computers, inkjet printers have come be used widely for printing images on paper, film or cloth not only in offices but also at home.
Inkjet recording methods include a method of discharging liquid droplets by applying pressure through a piezo element, a method of generating bubbles in an ink by heat, thereby discharging liquid droplets, a method of using ultrasonic waves, and a method of attracting and discharging liquid droplets by electrostatic power. As an ink composition for such inkjet recording, water-based inks, oil-based inks or solid (melting type) inks are employed. Of these inks, water-based inks are mainly used in view of productivity, handling properties, smell and safety.
Colorants to be used in these inkjet recording inks are required to have a high solubility in a solvent, permit high-density recording, provide a good hue, have excellent fastness against light, heat, air, water and chemicals, exhibit good fixation to an image receiving material and cause less blurring, have excellent storage stability in the form of an ink, have no toxicity, have a high purity and be available at a low cost. However, it is extremely difficult to find colorants capable of satisfying these requirements at high levels.
Various dyes and pigments have already been proposed and practically used as colorants for inkjet recording but, colorants capable of satisfying all of these requirements have not been found yet. Dyes and pigments which have conventionally been well known and have respective Color Index (C.I.) numbers cannot simultaneously attain both the hue and fastness required for an inkjet recording ink.
As a black dye for inkjet recording, food black dyes, naphthol direct azo dyes and acid azo dyes have widely been known so far.
Typical examples of the food black dyes include C.I. Food Black 1 and C.I. Food Black 2. A description on the using technique of these dyes for inkjet black ink can be found in Japanese Patent Laid-Open No. 36276/1990, Japanese Patent Laid-Open No. 233782/1990 and Japanese Patent Laid-Open No. 233783/1990.
The using technique of C.I. Acid Black 2, C.I. Acid Black 31, C.I. Acid Black 52, C.I. Acid Black 140 and C.I. Acid Black 187 as an acidic azo dye is described in Japanese Patent Laid-Open No. 108481/1985, Japanese Patent Laid-Open No. 36277/1990 and Japanese Patent Laid-Open No. 36278/1990, while that of C.I. Direct Black 9, C.I. Direct Black 17, C.I. Direct Black 38, C.I. Direct Black 51, C.I. Direct Black 60, C.I. Direct Black 102, C.I. Direct Black 107, C.I. Direct Black 122, C.I. Direct Black 142, C.I. Direct Black 154 and C.I. Direct Black 168 as a direct azo dye is described in Japanese Patent Laid-Open No. 139568/1981, Japanese Patent Laid-Open No. 285275/1986 and Japanese Patent Laid-Open No. 106974/1991.
It is usually very difficult to produce the color hue of black only by a single dye so a short-wavelength dye is preferably used in combination.
As this short wavelength dye, direct azo dyes and acid azo dyes are widely known, and the using technique of C.I. Acid Yellow 17, C.I. Acid Yellow 23, C.I. Acid Yellow 49 and C.I. Acid Yellow 194, and C.I. Direct Yellow 86, C.I. Direct Yellow 120, C.I. Direct Yellow 132 and C.I. Direct Yellow 144 for inkjet black ink is described in Japanese Patent Laid-Open No. 97541/1995, International Publication No. 97/16496, Japanese Patent Laid-Open No. 158560/1998 and Japanese Patent Laid-Open No. 12514/1999.
The dye of the invention is superior in fastness to these known dyes and using technique of the dye of the invention as an inkjet black ink is described in Japanese Patent Application No. 2002-113460.
The present inventors have proceeded wit an investigation on inkjet inks using a dye. As a result, it has been found that an aqueous black ink has such a drawback as low image durability; and that different from carbon black, almost no black dyes are capable of producing black images having high print quality required for printing of characters by the single use of them and several dyes different in absorption spectrum must be mixed. It has also been found that if some dyes are mixed, the quality of an image including therein a black image is impaired remarkably by a difference in fading rate among them, or by a change in absorption waveform of the dye having broad absorption characteristics owing to fading. Since the solid concentration of a water soluble black ink tends to become high because of necessity of having absorption characteristics sufficient to cover a wide ranging wavelength region, a solubility of the dye in a water-based solvent is important. The dye, to which a dissociation group has therefore been introduced, is frequently used in the form of a metal salt. By the existence of a heavy metal, damage of a head is accelerated and such a deterioration is marked particularly under high humidity conditions.